The stator cores of electrical generators and other electrical machines are made up of a stack of several, for example, thousands of individual steel sheets or laminations. The thickness of an individual sheet is typically measured in thousandths of an inch. Each of the laminations is coated with a thin layer of electrical insulation to insulate it electrically from its neighbors. This insulation prevents the alternating magnetic flux in the stator core from inducing eddy currents between laminations. If the insulation between adjacent laminations becomes damaged during assembly, operation or maintenance, a conducting path may be formed through which currents are induced by the alternating flux. These conducting currents create “hot spots” which, if undetected, can result in failure of the machine.
One type of apparatus suitable for detecting such stator core hot spots is an Electro-magnetic Core Imperfection Detector (“EL CID”). An EL CID detects potential hot spots electromagnetically by exciting the stator core and then measuring any resulting fault currents flowing through a damaged region. Typically, the EL CID's sensors are held against the stator core laminations and transported along the length of the stator core. As the sensors scan axially along the length of the core, they produce an analog signal having a magnitude proportional to detected fault currents. By examining a plot of the magnitude of the analog signal versus the sensor distance along the length of the core, operators are able to detect the location of a damaged insulation.